Flight simulator and flight simulator ROS are two different things. Let us put is it this way, say that through this simulator you wanted to model a car. Flight simulator would be the idea of the car (the interaction of the wheels, how much torque the motor is supposed to give etc...) and Flight simulator ROS would be the physical incarnation of the car. Basically Johaness wrote the flight simulator to contain all the logic of the simulator which can then be interfaced through any simulator or interface he chooses. There is already an example of the reusibility of the simulator in the gitlab. In our case flight simulator ros is the interface but initially Johanes tried to migrate the simulation to ros 2 (flight project ros 2) but then gave up, this did not impact the fundamental logic of the simulation as the two are completely decoupled.